1. Field of the Invention
The present invention relates to a lithographic apparatus, a projection system and a damper for use in a lithographic apparatus and a method for manufacturing a device.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
In general, there is a continual drive in the industry to provide lithographic apparatus capable of producing ever smaller pattern features. Consequently, there is a corresponding continual drive to improve the control of the lithographic process. In addition, as discussed above, it has been proposed to use a lithographic apparatus having a programmable patterning device. Such a lithographic device has improved flexibility because it is easier to change or adjust the pattern to be formed on a substrate. However, the use of a programmable patterning device may impose even more stringent requirements on the control of the lithographic process. For example, at least partly because of the constraints on the size of a programmable patterning device that can be formed, the area on a substrate that can be exposed at a given instant by a lithography apparatus using a programmable patterning device is significantly smaller than the corresponding area in a conventional lithographic apparatus. Consequently, in order to provide a lithographic apparatus having a sufficient throughput, each portion of a substrate may be exposed by a relatively small number of pulses of a pulsed radiation source compared to that of a conventional lithography apparatus. For example, whereas in a conventional lithography apparatus, each part of the substrate may be exposed by an exposure consisting of approximately 50 to 100 pulses of a pulsed radiation source, in a lithography apparatus using a programmable patterning device, it may be necessary to expose each portion of the substrate with only a few pulses of the pulsed radiation source and, possibly, only a single pulse. This results in a requirement for greater positional control of the optical elements in the projection system. Specifically, errors in the position of the optical elements within the projection system result in errors of the position of the pattern projected onto the substrate. Where a large number of pulses of radiation are used to expose a pattern, errors of the position of the optical element, for example caused by vibrations, are averaged out. Accordingly, vibration of the optical elements does not cause a shift of the image formed on the substrate, but merely slight blurring of the image. In contrast, if only a single pulse or a small number of pulses is used to form the pattern on the substrate, e.g., such as may be the case in a lithographic apparatus using a programmable patterning device, there are not sufficient number of pulses that slight positional errors are averaged out. Accordingly, errors in the position of the optical elements within the projection system result in displacement of the image formed on the substrate, e.g., overlay errors. Accordingly, it is necessary to improve the accuracy of the positioning of the optical elements within a projection system.
Previously, the accuracy of the positioning of the optical elements in a projection system has been improved by the provision of tuned-mass dampers or servos with accelerometer/geophone feedback which ensure that the optical elements remain substantially stationary even if the housing of the projection system is disturbed, for example if vibrations are transmitted into the projection system. However, such configurations may be relatively massive and/or may require too much space around the optical element. Furthermore, such arrangements may not provide sufficient accuracy of control of the patterned beam of radiation that is projected onto the substrate.